This invention relates to tissue distractors, more particularly to a mechanism for powering an auto-extensible tissue distractor, such as a bone fixator.
Since the turn of the last century when Ilizarov discovered that new bone and soft tissue is regenerated under the effect of slow and gradual distraction, external fixation has been utilised in the treatment of various bone conditions. Limb length differences resulting from congenital, developmental, post-traumatic or post-surgical causes may be treated in this manner. The procedure also lends itself to the treatment of congenital deformities, post-traumatic bone deformities, non-healing fractures and bone loss from tumour, trauma or infection.
Traditionally an external bone fixator has been used which comprises a framework of metal rings connected by rods, whereby each ring is connected to the bone by a plurality of wires under tension or by pins. Titanium pins may be used to support the bone. Presently, a wide variety of designs of fixator are available and are suitable for withstanding the forces imposed by the full weight of the patient.
In surgical limb lengthening, the bone is subjected to osteotomy so as to sever it into two or more parts before the fixator is attached to the severed parts of the bone. In the course of the operation the surgeon will attach at least one pair of pins to each of the severed parts of the bone and then join the pins externally of the patient""s limb by means of a rod or rods. Generally there is at least one rod on each side of the limb. Just a few days after surgery the patient is encouraged to resume normal use of the limb in order to maintain joint flexibility and to facilitate muscle growth to match the osteogenesis.
Approximately one week after the surgery to fit the fixator, manual adjustments are commenced in order to lengthen the rods equally so as to separate the severed ends of the bone at a rate of about 1 mm per day. An increase of more than about 1 mm per day results in a slowing of the osteogenesis and an increase of less than about 1 mm per day can result in premature consolidation.
In surgical limb straightening the bone can be severed completely or partially. If the bone is completely severed, then the rod or rods on one side of the limb may be lengthened at a greater rate than the rod or rods on the other side thereof. Alternatively the bone can be partially severed according to a technique known as open wedge osteotomy, in which case a bone fixator may be needed only on the side of the bone in which the cut has been made by the surgeon.
It has further been found that osteogenesis proceeds more satisfactorily if frequent small adjustments in bone length are made by distraction rather than larger less frequent adjustments of bone length. Hence adjustments of about 0.25 mm every 6 hours are recommended. This places a burden upon the patient and carer to conform to a routine which can be very disruptive to day to day life.
It is very common for patients to experience a great deal of pain each time that the fixator is incrementally lengthened. This can make the four times daily lengthening procedure a traumatic experience both for the patient and for the patient""s carer, particularly if the patient is a young child. Since the entire bone lengthening or straightening process can last from three to six months this can impose a continuing great strain not only on the patient but also on those caring for the patient. Moreover this procedure tends to lead to very high complication rates so that it is not uncommon for the complication rate to be as high as about 200% which means that each patient on average experiences at least two incidents during a course of bone lengthening or straightening treatment requiring a return to hospital, possibly for further surgery.
Another problem with external bone fixators is that there is a significant risk of infection arising at the site of each pin or wire.
It has been proposed to utilise gradual motorised distraction in which a typical procedure could involve applying a very small incremental lengthening over 1000 times per day which still achieves an average bone lengthening rate of about 1 mm per day.
It would be desirable to provide an auto-extensible tissue distractor, such as a bone fixator, which can effect substantially continuous distraction of bone or other living tissue throughout the day, even while the patient is asleep, thereby avoiding for the patient the pain associated with a several times daily incremental lengthening of the bone or other tissue. It would also be desirable to provide a bone fixator of sufficiently compact size to enable its use as an internal bone fixator, either attached surgically to the outside of the patient""s bone or even to the inside the patient""s bone, whereby the fixator can remain in situ after the bone lengthening or straightening process has been completed. It would be further desirable to provide a bone fixator which effects continuous bone distraction and which is suitable for external use or for implantation within or adjacent the bone to be lengthened.
The present invention accordingly seeks to provide a mechanism for use with a tissue distractor which enables tissue distraction to be effected substantially continuously throughout the day and night during the entire tissue distraction process. It further seeks to provide a mechanism for powering an auto-extensible bone fixator. It also seeks to provide such a mechanism which can be used with an external fixator. Another objective of the invention is to provide a mechanism for powering an auto-extensible internal tissue distractor. In addition the invention seeks to provide an auto-extensible bone fixator which avoids for the patient the pain associated with a four times daily incremental lengthening of about 0.25 mm by effecting substantially continuous bone distraction throughout the patient""s waking hours (and possibly also during the patient""s sleeping hours). A further objective of the invention is to provide a mechanism for powering a tissue distractor, such as a bone fixator, so as to cause it to extend in a series of very small increments so as to avoid, for a patient fitted with the tissue distractor, the experience of pain associated due to the extension of the tissue distractor. Yet another objective of the invention is to provide a compact bone fixator which can be used as an internal bone fixator, for example by being attached surgically to the outside of the patient""s bone or by being implanted inside the patient""s bone. In addition the invention seeks to provide a compact design of bone fixator which effects substantially continuous bone distraction and which is suitable for external use or for implantation within or adjacent a bone to be surgically lengthened or straightened. An additional objective is to provide an auto-extensible mechanism for a bone fixator which can automatically lengthen at an essentially continuous and controllable rate while measuring both the rate of lengthening and also the load being imposed on the bone being lengthened and its associated soft tissue.
According to the present invention there is provided a mechanism for powering an auto-extensible tissue distractor comprising:
an elongate member having a proximal end, a distal end, and a longitudinal axis;
at least one movable device movably mounted on the elongate member for movement therealong in a direction away from the proximal end towards the distal end, said movable device comprising:
a first crawler portion operatively engaged with the elongate member and having a first non-return means for preventing movement of the first crawler portion at least in a direction towards the proximal end;
a second crawler portion operatively engaged with the elongate member and disposed nearer the distal end than the first crawler portion, the second crawler portion having a second non-return means for preventing movement of the second crawler portion at least in a direction towards the proximal end;
piezoelectric drive means adapted to undergo elongation by a discrete incremental amount in an elongation direction upon application of a selected voltage potential thereto and at a rate of elongation dependent upon the rate of increase of the voltage applied thereto, the piezoelectric drive means being operatively mounted between the first and second crawler portions with its elongation direction aligned substantially parallel to the longitudinal axis of the elongate member so that, upon applying the selected voltage potential thereto at a predetermined rate of increase of applied voltage with the first non-return means preventing the first crawler portion from moving in a direction towards the proximal end of the elongate member and with the second non-return means permitting movement of the second crawler portion towards the distal end, the second crawler portion is caused to move at a predetermined rate of elongation a distance corresponding to the incremental amount towards the distal end, and so that, upon removal of the selected voltage from the piezoelectric means with the second non-return means preventing the second crawler portion from moving towards the proximal end and with the first non-return means permitting the first crawler portion to move towards the distal end, the piezoelectric drive means reverts to its original length and thereby moves the first crawler portion a distance along the elongate member towards the distal end corresponding to the incremental amount;
voltage generating means for applying a voltage potential to the piezoelectric means; and
control means for repeatedly applying the selected voltage potential at the predetermined rate of increase of applied voltage to the piezoelectric means to cause the movable device to move in incremental steps along the elongate member in a controlled manner.
Throughout this specification and claims the terms xe2x80x9cincreasexe2x80x9d and xe2x80x9cincreasingxe2x80x9d, when used in relation to voltage potentials, refer to the magnitude of the voltage and do not necessarily imply, for example, that the voltage potential is increasing from 0 to +1000 v but also include within their meanings the case where the voltage potential is decreasing from 0 to xe2x88x921000 v, for example.
In a preferred form of mechanism according to the invention the movable device includes a rigid outer tubular member surrounding the first crawler portion, the second crawler portion, and the piezoelectric means. Moreover the piezoelectric means may be interposed between the first crawler portion and the second crawler portion for causing separation of the first crawler portion and the second crawler portion longitudinally of the elongate member.
In such a mechanism the first crawler portion may include a first substantially frustoconical recess whose axis is substantially coaxial with the longitudinal axis of the elongate member, and the first non-return means may comprise a spring loaded collet arranged to seat in the first substantially frustoconical recess to exert a braking action on the first crawler portion, the mechanism further comprising a first supplementary piezoelectric means for moving the collet away from the first substantially frustoconical recess to release the braking action of the first non-return means. Typically the first supplementary piezoelectric means comprises a tubular piezoelectric actuator. Alternatively the first supplementary piezoelectric means may comprise a pair of rod-like piezoelectric actuators. In addition the second crawler portion may include a second substantially frustoconical recess whose axis is substantially coaxial with the longitudinal axis of the elongate member, while the second non-return means comprises a spring loaded collet arranged to seat in the second substantially frustoconical recess to exert a braking action on the second crawler portion, the mechanism further comprising a second supplementary piezoelectric means for moving the collet away from the second substantially frustoconical recess to release the braking action of the second non-return means. Such a second supplementary piezoelectric means may comprise a tubular piezoelectric actuator or a pair of rod-like piezoelectric actuators.
In such a mechanism the piezoelectric means may be tubular.
In an alternative preferred embodiment the first non-return means and the second non-return means each comprise sprags which brake movement towards the proximal end of the elongate member and which permit movement only in a direction towards the distal end of the elongate member. In such a mechanism the second crawler portion may be held captive on the first crawler portion and be spring biased away therefrom, while the piezoelectric means comprises a pair of rod-like piezoelectric actuators.
In yet another preferred form of mechanism according to the invention the second crawler portion is held captive on the first crawler portion and is spring biased away therefrom, and the piezoelectric means comprises a tubular piezoelectric actuator surrounding the elongate member. In such a mechanism the first crawler portion may comprise a collar having a pair of free ends, the collar being arranged to grip the elongate rod to brake movement of the first crawler portion relative to the elongate rod, and brake release means comprising a transversely mounted piezoelectric means arranged to separate the free ends of the collar to release the braking action thereof on the elongate member. In this case the second crawler portion may comprise a collar having a pair of free ends, the collar being arranged to grip the elongate rod to brake movement of the second crawler portion relative to the elongate rod, and brake release means comprising a transversely mounted piezoelectric means arranged to separate the free ends of the collar to release the braking action thereof on the elongate member.
In a still further form of mechanism according to the invention the elongate member comprises a tubular portion, the first crawler portion comprises a first plug slidably disposed within the tubular portion and connected to an outer ring member which surrounds the elongate member, the second crawler portion comprises a second plug slidably disposed within the tubular portion and connected to a main body portion surrounding the elongate member, the first non-return means and the second non-return means each comprise respective sprags, the piezoelectric drive means is disposed within the tubular portion between the first and second plugs, and spring biassing means are provided between the first crawler portion and the second crawler portion and arranged to undergo loading while the first non-return means prevents movement of the first crawler portion during movement of the second crawler portion and for the loading to be released while the second non-return means prevents movement of the second crawler portion so as to cause the first crawler portion to move towards the distal end of the elongate member.
In the mechanism of the invention the control means may include a load sensor, such as a load cell, for measuring the load applied by or to the piezoelectric drive means. Such a load cell may comprise a piezoelectric crystal upon which the piezoelectric drive means is arranged to impinge.
In a preferred form of the invention the control means includes a microprocessor, amplifier means operatively connected to the piezoelectric drive means and under the control of the microprocessor for applying at least one voltage potential signal to the piezoelectric drive means at a selected rate of increase thereof for causing the movable device to move in incremental steps along the elongate member in a controlled manner, feedback amplifier means connected to the load sensor and arranged to receive a feedback signal therefrom, the feedback amplifier means being operatively connected to the microprocessor for supplying thereto a feedback signal in dependence on the load sensed by the load sensor and the microprocessor being arranged to receive the feedback signal from the feedback amplifier means and to adjust the rate of increase of the at least one voltage potential signal so as to ensure that the load exerted by or on the piezoelectric drive means and sensed by the load sensor does not exceed a predetermined value. Preferably the microprocessor is arranged to store data indicative of the at least one voltage potential signal, of time, and of the feedback signal from the feedback amplifier means. Typically the at least one voltage potential signal comprises a d.c. voltage potential signal which is typically in the range of from about 100 v to about 1000 v. However, if desired, the at least one voltage potential signal may comprise an oscillatory signal having a frequency of from about 5 Hz to about 2 kHz, e.g. about 1 kHz, having an amplitude of from about 1 xcexcm to about 10 xcexcm imposed upon the d.c. voltage potential signal. In this last mentioned case the amplitude of the oscillatory signal must not exceed the overall extension of the piezoelectric drive means caused by the d.c. voltage potential at the time so as not to damage the piezoelectric drive means. The oscillatory signal can be superimposed on the d.c. voltage potential signal either while the magnitude of the voltage potential is increasing or while the d.c. voltage potential is held substantially constant either before or after the piezoelectric drive means has reached its maximum extension during an incremental step.
It will normally be preferred for the microprocessor to be connected to an input/output device by means of which set point values can be input into the microprocessor from an external input device and data stored in the microprocessor can be downloaded to an external source. Such set point values can, for example, include set point values indicative of at least one of the following items:
(i) the rate of increase of d.c. voltage potential with time applied to the piezoelectric drive means thereby to determine the rate of extension thereof;
(ii) a functional relationship involving the rate of increase of the d.c. voltage potential applied to the piezoelectric drive means and also the load sensed by the load cell;
(iii) the maximum d.c. voltage potential to be applied to the piezoelectric drive means thereby to determine the size of each said incremental step;
(iv) the maximum load sensed by the load sensor permitted to be applied by or to the piezoelectric drive means thereby to determine the load imposed on the bone callus and associated soft tissue;
(iv) the maximum time period during which the movable device is permitted to move thereby to determine the maximum distance through which the movable device is permitted to move along the elongate member;
(v) the maximum distance through which the movable device is permitted to move along the elongate member.
Desirably the mechanism is arranged so that the incremental steps range from about 5 xcexcm to about 120 xcexcm, preferably about 10 xcexcm to about 120 xcexcm, for example, from about 40 xcexcm to about 80 xcexcm.
The mechanism will typically be adjusted so that the control means causes the movable device to move along the movable member at a rate of about 1 mm per day.
The invention also extends to an auto-extensible tissue distractor, such as a bone fixator, fitted with a mechanism of the above described type.